Microstructure And Mechanical Properties Of Mg-Y-Nd-Gd-La-Zr Alloys

Magnesium alloy,as a green engineering material for the 21 st century,has important application value in many fields such as aerospace,transportation,and electronic products.However,the poor heat resistance and low strength of magnesium alloys have limited its engineering application in the aerospace field,Therefore,the development of magnesium alloys with good mechanical properties at room temperature and high temperature,low rare earth content,and low price have become one of the main directions in the current research field.In this paper,the independently designed Mg-4Y-2Nd-1Gd-x La-0.5Zr alloy is subjected to solid solution and aging treatment.The optical microscope(OM),scanning electron microscope(SEM),energy spectrometer(EDS),and X-ray diffractometer are applied.(XRD),universal tensile testing machine,and other analytical testing methods have been used to study the effects of the addition of La on the microstructure,mechanical properties,fracture mechanism,work hardening,and strengthening mechanism of as-cast,solid-solution,and aging alloys.Optimize the best heat treatment process and composition of the alloy.The results show that among the four Mg-4Y-2Nd-1Gd-x La-0.5Zr as-cast alloys,with the addition of La from 0wt% to 1.0wt%,the grain size of the alloy decreases first and then increases.When the additive amount is 0.5wt%(WNG42105 alloy),the alloy has the finest grains,yield strength,tensile strength,and elongation are 170 MPa,215MPa,and 6.3%,respectively,and the hardness is 74 HV,which is higher than that of the other three alloys,and work hardening effect is good,it can be normally used at 200?.The optimal solution treatment process of Mg-4Y-2Nd-1Gd-x La-0.5Zr alloy system is 520?-10 h.After this process,the grain size of the four alloys first increases and then decreases with the addition of La.The second phase of the WNG42105 alloy is basically completely soluted,and only a small amount of the second phase exists in some grain boundaries.The yield strength,tensile strength,and elongation are 175 MPa,225 MPa,and 8.2%,respectively.The hardness is 77 HV,which is higher than other three alloys.The cooling method after solution treatment also affects the microstructure and mechanical properties of the alloy,and the hot water cooling effect is the best.Compared with the other three alloys,the distribution of ?”and ?'precipitated phases is more uniform and dense under the peak ageing at 200?.When aged at 200?,225?,and 250?,the mechanical properties at the peak age of 200? of WNG42105 alloy are the best.The tensile and yield strength are 298 MPa and 241 MPa,respectively,the elongation is 4.2%.and the hardness is 108 HV The strengthening mechanism is mainly grain boundary strengthening and precipitation strengthening.With the aging time increasing,the precipitates free zones at the grain boundaries of WNG42105 alloy gradually get widen.At high temperature aging,the main precipitation phase of alloy within 400? is the ? phase,but the ? phase above 450? cannot exist stably.When the 200? aging WNG42105 alloy is stretched at 200? and 250?,the mechanical properties are not sensitive to the strain rate.When stretched at 300?,the yield strength and tensile strength decrease,and the elongation increases as the strain rate decreases.The yield strength can still be maintained at about 250 MPa,and the heat resistance is excellent.When stretched at 350? and 400?,the yield strength and tensile strength of the alloy decrease as the strain rate decreases,but the elongation compared to room temperature is greatly improved.During the compression process at room temperature,the yield strength of the peak aged Mg-4Y-2Nd-1Gd-x La-0.5Zr alloy at 250?,225?,and 200? decreased sequentially,but were higher than those of the as-cast and solid-solution alloys.The elongation of the solid solution alloy is higher than that of the as-cast and three aged alloys.During compression,as the strain rate increases,the yield strength of the alloy increases,but the elongation decreases,and the compressive strength is not sensitive to the strain rate.In summary,when addition of La is 0.5wt%,the comprehensive properties of Mg-4Y-2Nd-1Gd-x La-0.5Zr alloy in the as-cast,solid solution,and aging states are higher than the other three alloys.After 520?-10 h solid solution cooling,hot water cooling,and 200?-34 h aging optimization process,the comprehensive mechanical properties and heat resistance are significantly improved,which can play an important role in some heat-resistant structural parts in the aerospace field.